We always think we have seen it all, but occasionally something will come along that makes us scratch our head and say "huh."
Case in point: we get a sample in and it tests a high positive, we notify the producer and advise they get a follow up sample in 2 to 3 weeks.
Now, when we tell a producer we have a positive test, it's positive. Our head lab tech Sonia will run positives over and over again. If she gets a positive she immediately re-runs it twice, then she'll run it again the next day on two different tests.
Fast forward two weeks and we get the follow up sample. Sonia runs it and it comes out negative. Wait, WHAT!? She ran it again. Negative. We called in the head honcho, Dr. Bill and he runs it that afternoon on two different tests and it's still negative. At this point we are trying to found out how we messed up 5 different times running the first sample.
So we send off both samples to a state lab to get Immunohistrochemistry tests, Polymerase chain reaction tests and virus isolation tests done. All the tests came back the same: the first sample is positive and the second sample is negative. We call the producer and found out he took the samples from different ears of the same animal, so we started thinking, can a PI give different values depending on the ear you get the sample from?
That simply can't happen, you may get some variation in color but not strong positive to negative. We know the animal in question is in the PI pen, so we wondered, is he having an inhibitory effect with another BVD substrain that is causing his to test negative? We know this can effect ELISA and PCR tests.
Before we sent off 30+ samples of PI's to the National Animal Disease Center we decided to have the state lab run some DNA tests.
We found the problem: the samples came from two different animals. The first sample was a true PI, the second animal was a negative in the PI pen.
Make sure your producer is giving you the correct sample on a follow up test.